Films, membranes, sheets, boards, adhesives, coatings, particle-laden fluids, blown fibers and the like are formed by an extrusion process including an extrusion die (also known as a “flat die” or, simply, a “die”). The extrusion die includes a cavity into which a prepared fluid or paste (such as a polymer melt) flows. The die cavity is configured to create substantial cross-directional (CD) uniformity in flow rate and temperature. Sometimes, additional criteria, such as residence time, shear rate, and shear history, are important, because these criteria influence the molecular weight and poly-dispersity of the material. After flowing through the die cavity, the fluid exits the die through a gap opening as a uniform sheet or through a row of orifices as liquid filaments ready for further treatments.
Designing a die cavity geometry requires one to make use of rheological principles and knowledge of the properties of the material. A wide variety of materials may be used in the extrusion process. Newtonian fluids are those materials having fluid properties that are unaffected by flow parameters. There is no shortage of die designs for extruding Newtonian fluids.
Some of the materials have large and complex molecular structures and properties that change with surroundings and treatments, such as molten thermoplastic and thermo-set polymers. Their flow behaviors are of the non-Newtonian type, in which material properties (such as viscosity, density, and molecular weight) and flow dynamics (such as shear rate, shear history, temperature, thermal history, heat transfer rate, and pressure) influence each other. For non-Newtonian fluids, some extent of material degradation at the end of processing is unavoidable and irreversible. However, such degradation may be minimized and made uniform by utilizing a properly designed die. The fabrication of dies also requires specialized skill, experience, and equipment.
Unfortunately, for molten polymers and other non-Newtonian fluids, there are few suitable die choices. FIG. 1 illustrates several conventional die types, including a simple cavity die 11, a slot die 12 having a T-shape, a linearly tapered die 13, and a fishtail die 14. FIG. 3 illustrates a so-called “coat hanger” die 3, which is commercially popular.
FIGS. 2A and 2B illustrate an extrusion assembly for producing wide webs. For expedience and savings, several narrow dies may be arranged abutting one another in their width direction, as taught by U.S. Pat. No. 7,438,544 and U.S. Pat. No. 7,690,902. In such an assembly, fluid materials are introduced via a melt inlet 21. Fluid delivery pipes 24, 25 direct the fluid materials through melt pumps 22, which are powered by melt pump motors 23. The melt pumps 22 push the fluid material through a series of melt reservoir supply pipes 26 and into a melt reservoir 27. From the melt reservoir 27, the fluid materials are extruded as a film, a web, or a plurality of fibers. Such an assembly has sub-par uniformity, and capital and operating costs associated with such an assembly are high. Thus, this technique has limited practical merit.
FIG. 3 is one half of a conventional coat hanger die 3 having a melt inlet 31 and a coat hanger flow channel 32 that surrounds a land area 33. The fluid material flows through the arc-shaped flow channel 32 and is extruded through an extrusion trough 34. The die halves are secured by assembly fasteners 35, such as bolts or screws, which are located upstream of the flow channel 32 and outside of the land area 33. One representative example of such a conventional coat hanger die is found in U.S. Pat. No. 4,285,655.
The formula for the geometry of the coat hanger die is derived from solving differential equations of the polymer's rheological formula while incorporating boundary conditions and desired states, such as nominal melt velocity at the center, equal flow and shear rates in the cross-machine direction, equal residence time and shear history along each streamline, and lack of slip at the walls. As a result, the shape looks like a coat hanger (thus, the name) with a round- or teardrop-shaped flow channel (32). After simplification and approximation, the radius of the channel Rx and its position Hx can be expressed by the following short formulas:
            R      x        =                            R          0                ⁡                  (                      x            L                    )                            1        /        3                        H      x        =                            H          0                ⁡                  (                      x            L                    )                            2        /        3            In these formulas, x represents a location along the width of the die, as measured from an outermost edge of the die. The “L” in the formulas is a constant for a given die, which is equal to one-half of the maximum width of the die. The maximum radius Ro and the maximum die height Ho are constants usually recommended by experienced die builders. These formulas have been widely used by extrusion industries for several decades.
Worldwide users have had considerable experience with coat hanger dies, and they generally have expressed the following comments or suggestions for improvement. In no particular order, their comments include:
1. The coat hanger die is bulky and heavy. For example, a die having a width of one meter weighs about one ton.
2. The coat hanger die is expensive and difficult to make and repair. Typically, the coat hanger die is made from vacuum furnace-fused tool steel (for purity), which has been treated for chemical resistance and hardness. The machining and polishing of three-dimensional cavities within the die require sophisticated equipment and skills.
3. The land area 33 of the coat hanger die 3 cannot accommodate bolts or screws. Lacking support, the coat hanger die 3 bulges under pressure to a shape that, when exaggerated, resembles a clamshell. The resulting clamshell shape reduces the uniformity of all flow parameters and is particularly problematic for wide dies. It is very difficult to monitor and alleviate the “clamshell effect,” which is a function of pressure and die's design and age.
4. Overheating, uneven heating, over pressure, and mechanical impact or scratches can damage a die and upset production.
5. Coat hanger dies with greater heights Ho yield better uniformity results. But die height drives up die size, weight, and cost. Compromises must be made in the use of such wide dies with the number of compromises being higher with increasing die widths.
6. In reality, coat hanger dies are not sufficiently indiscriminating to different polymeric materials. While some material changes cause only minor differences in resulting uniformity, others may not be so negligible. The lack of uniformity causes concern to production lines that switch raw materials often, since wide dies are too inconvenient to be moved in and out from a production line frequently and are too expensive to remain idled.
7. The polymer species most commonly used for blown fibers are polypropylene, polyethylene, polyethylene terephthalate (PET), and linear polyamides, which are more sensitive to heat and shear than other polymers. When making fibers, these polymers need to be processed with higher temperature and lower viscosity than are used in film/sheet extrusion, in order to get acceptable throughput from tiny orifices. Issues such as thermal degradation, shear thinning, and residence time effect are more critical in making fibers than films. Thus, the need for a suitable die is more critical, and achieving a satisfactory result takes more trial-and-error effort.
8. In the mathematical derivation of the coat hanger-shaped geometry, assumptions and approximations lead to inaccuracy. For example, the streamlines in a large coat hanger's land area may not always be straight and parallel; actual melt viscosity may deviate from the simplified laws (such as Power law and Eyring law); and melt pressure and speed may also affect viscosity.
9. For the above reason, die makers must test and analyze the material(s) to be extruded with their proprietary computer programs before they design the die cavity geometry. Similarly, polymer suppliers tailor their resins or additives with their proprietary formulations to optimize the polymer's performance in coat hanger dies. Such customization increases cost, time, and inconvenience.
10. Not reflected in its mathematical formula, coat hanger dies have been empirically found to have varied uniformity results for changes in melt temperature, flow rates, and polymer species. The deviations are more pronounced at and near the edges.
11. The two edges of the extruded sheet are routinely too light or too thick, even at normal operating conditions. As a result, manufacturers treat the edges as off-quality. As such, the manufacturers trim off the edges and recycle the polymeric material.
12. During maintenance, a purging compound is extruded through the entire cavity inside the flat die. Because the purging compound is vastly different from the polymer melt in both composition and flow rate, the purging compound cannot be expected to scour the die cavity uniformly.
13. Intervention systems (such as automatic lip, choker bar, and computerized temperature manipulation systems) are commonly needed to improve the flow profile of wide dies, but at the cost of shear rate and shear history uniformity. These remedial equipments and operations also add considerable cost, skill, labor, maintenance and downtime.
Despite its challenges, the coat hanger die design represents a substantial advantage over its predecessors. There have been few challengers to this die type. One such challenger is F. Rothemeyer, whose proposed die assembly was published originally in German in an article entitled “Bemessung von Extrusionswerkzeugen” in the journal Maschinenmarkt, Vol. 39, pages 679-685 (1979) and whose work was subsequently described in English by W. Michaeli in Extrusion Dies for Plastics and Rubber, 2nd edition, Hanser Publishers, pages 147-152 (1992). A sketch of this die is shown in FIG. 4.
The Rothemeyer die 4 is constructed with a three-dimensional shape that ensures each flow path within the die cavity 44 has an equal length, such that they have an identical flow rate and flow history. The polymer melt enters the die 4 through a melt inlet 41 and travels through the die cavity 44 between the lower half 42 of the die and the upper half 43 of the die. The polymer exits through an extrusion trough 45. The performance of the die 4 is independent of the material being processed and the operating parameters of the extrusion system.
Many of the drawbacks of the proposed die are associated with its large three-dimensional shape. The material requirements and steel waste are also high. The cavity 44 is difficult to machine, and the labor requirements are high. The programming and set-up for CNC milling are time-consuming. Finally, it is possible that the “clamshell” concern might actually be worse, not better, than that of existing coat hanger dies. For these reasons, there appears to be no evidence of the commercial use of this die.
Therefore, an improved die design would benefit a vast number of users and end products. It would be even more useful if the die design is capable of processing Newtonian and non-Newtonian fluids equally well. It is a scope of this disclosure to provide as many as possible improvements or reliefs to the aforementioned needs.
A second, but equally significant, challenge unmet by conventional dies is the ability to withstand pressures associated with the production of very small diameter (e.g., nano-scale) fibers. A micron is one-millionth of a meter. The term “nano-fibers” is used to describe fibers having an average diameter size measured in nanometers (nm), or one-billionth of a meter. The term “submicron fibers” refers to fibers having an average diameter size of between 500 and 999 nm. Nano-fibers having an average diameter of between 100 and 500 nm and submicron fibers having an average diameter of between 500 and 999 nm are of particular commercial interest and are difficult to obtain using presently available dies and die tips.
Since the introduction of very small fibers, they have been used in numerous new applications, such as biomedical (e.g., synthetic tissues, organs, skin, blood vessels; wound healing; drug release; metal ion adsorption for detoxification), carriers for enzymes and catalysts, sensors, weapons and warfare, environmental protection, water/gas filtration and purification, personal protection/care, energy devices (e.g., lithium batteries, super capacitors, fast chargers, solar cells, fuel cells, hydrogen and natural gas storage/transportation, renewable energy harvest and storage, electric vehicles), electronics, membrane replacements, and the like.
In the production of very fine fibers, such as submicron and nano-fibers, melt-blowing competes with electro-spin technology. Electro-spin technology has successfully produced fibers as fine as 100 nm in tiny quantities, while the best available melt-blowing devices are capable of producing slightly coarser fibers ranging from 350 to 500 nm with greater economy. Accordingly, both technologies have a potential for advancement and would benefit from improvements thereto.
Recent studies and the inventor's experience suggest that the major impediments to creating melt-blown nano-fibers (less than 500 nm) are the currently available die and the die tip. Conventional dies and die tips used for extruding melt-blown fibers are unable to process melts of super low viscosity and are unable to withstand the extremely high pressures required for very small orifices. For example, conventional dies (such as those discussed above) and die tips are typically designed for pressures of about 600 psi to 900 psi (pounds per square inch), whereas pressures of 3,000 psi or greater may be necessary for producing very fine fibers. The low melt viscosity is needed for spinning super thin fibers, while the high pressure is required to afford a reasonable production rate. Conventional dies “clamshell” tendency worsens seriously with greater pressure and die width. Additionally, because many nano-fiber types must be made from difficult-to-process and rare polymer species, extrusion dies are needed that are indiscriminating to all materials in all process conditions. Current coat hanger dies fall short on all these critical requirements.
Therefore, another object of the present disclosure is to provide a die design capable of producing fibers of a very small diameter, including nano-fibers.
The problem with conventional die tips is that they simply do not have orifices that are small enough to produce the target fiber size. The present state-of-the-art die tip equally would benefit from improvements to address its own shortcomings. Such an improved die tip is subject of another disclosure by the present inventor, U.S. patent application Ser. No. 14/850,877, entitled “DIE TIP FOR MELT BLOWING MICRO AND NANO-FIBERS,” filed concurrently herewith and incorporated in its entirety by reference herein.